Quality quartz oscillators use a thermostated resonator. The resonator proper is usually mounted in a vacuum inside a sealed casing, which is placed inside an oven that includes a heating resistor or a power transistor and a heat conductive structure. The unit is insulated from the outside with polyurethane foam or an equivalent, or with a Dewar flask. The temperature is adjusted constantly with reference to a standard voltage. This kind of thermostat yields remarkable results with respect to frequency stability. However, it displays the disadvantage of being bulky, of consuming energy and of taking a fairly long time to set the temperature. Indeed, it is difficult to obtain proper operation of the oscillator within less than one hour after it is charged. The necessary power presently is about 15 watts during the start-up period, and it levels off at about 2 or 3 watts, depending upon the outside temperature (about 3 W at -50.degree. C. and 2 W at +25.degree. C.).
In order to remedy some of those disadvantages, it was suggested, especially in U.S. Pat. No. 3,431,392, that the quartz crystal should be heated with resistive layers directly deposited on the quartz cyrstal. These layers are made, for example, of nichrome, tantalum, or any other resistive metal.
It was also suggested in U.S. Pat. Nos. 2,969,471 and 3,121,153 that the piezoelectric crystal should be heated with radiating energy generated by heating resistors placed between the quartz crystal and concave reflectors.
These various solutions all display the inconvenience of using heating elements which de-gas in a vacuum, and thus altering the residual atmosphere inside the hood. Under these circumstances, the resonator ages badly. Futhermore, within those various solutions the temperature sensor used does not indicate the temperature of the vibrating portion of the resonator, but rather another temperature, for instance that of the edge of the resonator, or a segment thereof, or even that of another crystal which is mounted symmetrically in relation to the resonating crystal. Finally, the mechanical assemblies of the solutions which use reflectors are complicated or difficult to apply in practice.